Voice-operated echo suppressor



June 26, 1928.

0.5. BLACKWELL vozck OPERATED ECHO sprPnasson L Relay current 26 Relaynurran 2s 29 I Relay Energized I Relay Ewgizea? 5'0 3, y 4 Belay b'wzyized/ Kelaybzeryizar 3 52 1 L 55 Belay vurrazl/ Jielag wm'm IN VENTOR115194.41 0.1? fllawkwel ATTORNEY ing apparatus,

Patented June 26, 1928.

UNITED STATES PATENTS oF icE.

OTTO B. BLACKWELL, OF PLANDOME, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONEAND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

VOICE-OPERATED ECHO SUPPRESSOR.

Application filed August 3,1923. Serial No. 655,426.

This invention relates to transmission circuits of the four-wire typeand more particularly to arrangements for preventing echo eti'ects uponsuch circuits.

In telephone circuits of the four-wire type, difficulty has beenexperienced where the lines are of considerable length, due to so-calledecho effects. These effects arise from the fact that telephone currentsflowing over one of the two-wire lines compris ing the four-wire circuitare transmitted, due to unbalance at the terminal, back over thetwo-wire line transmitting in the oppo site direction. Since it takes afinite period of time for the current transmitted back to reach theoriginating station, especially where the circuits are loaded, thecurrent transmitted back produces a very disconcerting effect, similarto that of an echo.

In order to overcome this difficulty it has been proposed to arrangesome device responsive to voice currents transmitted over the one lineto disable the line. used for transmission in the opposite direction,there-- by preventing the transmission of echo currents over the reverseline. It has heretofore been the practice to arrange these echosuppressing. devices, one of which is associated with each line, at someintermediate station between the terminal stations, pref.- erably asnear as possible midway between the stations- Such an arrangement of theecho suppresswhile facilitating maintenance, has been found to have aserious disadvantage in that it prevents the subscriber who desires totalk in the reversedirection from getting immediate control of thecircuit after it has been used for transmitting in the oppositedirection. This is due to the fact that, owing to the time intervalwhich it' takes for an echo to be transmitted from the echo suppressorOn one side of the line to} the terminal and back to the point at whichthe opposite line is disabled, it has been found necessary to providethe echo suppressor with a delay .device whereby it maintains the linedisabled for a period longer than the time just referred to. Inaccordance with the present invention it is proposed to overcome thisditliculty by arranging the echo suppressing arrangement in two units,one located at or near each end' of the four-wire circuit. By thisarrangement, since the echo time is quite short from the point at whichthe voice currents operate the echo suppressor around the terminalconnections and back to the point at which the echo suppressor disablesthe return line, the echo suppressor may be made to restore the circuitto normal almost immediately upon the cessation of voice currents in onedirection. Consequently, the subscriber who desires to talk in the otherdirection can do so almost immediately.

The invention will now be more fully understood from the followingdescription when read in connection with the accompanying drawings,wherein Figure l is a simplified circuit diagram showing the arrangementof echo suppressors heretofore used. Fig.2 is a simplified diagramshowing the arrangement of the echo suppressor in accordance with. thepresent invention, Fig. 3 is a diagram showing somewhat in. detail theapparatus whereby the echoes are suppressed and Fig. 4 is a curveillustratingthe operation of the echo suppressor.

Referring to Fig. 1, a four-wire circuit comprising two-wire lines LEand LlV. is shown interconnected at the terminals to terminal lines Xand Y through hybrid coils 10 and 11 and balancing networks N and NOne-way amplifier schematically" indi cated at RE, and RE,, etc., andRVV and RYV, are includedin the two sides of the four-Wire circuit in avwell understood manner. In order to prevent voice currents transmittedover the line LE, for example, from being transmitted back over the lineLiV, due to unbalance at 11, an echo suppress-or arrangement isprovided, as indicated schematically at S l and SW, the arrangementbeing essentially a device which operates in response to voice currentsto actuate acontact device WlllCh will short-01rcuit or otherwisedisable the opposite l ne The details ofthe echo suppressor are,

illustrated in Fig. 3. Terminals 13 and '14 in this figure are connectedto the line over which voice currents are transmitted so that the highimpedance transformer 15 is bridged across the line without absorbingany appreciable amount of energy from the line. A vacuum tube amplifierA, of well known type is connected to the secondary of the transformerto amplify the voice currents received from the line. A detector orrecti-- fiervacuum tube D, of a type well known in the art, isassociated with the output side of the amplifier A for rectifying thevoice currents. Normally, the circuit of the detector D is so arrangedthat no current or, at any rate, an insufficient current flows throughthe" winding of the detector relay DR to operate said relay. When thevoice currents are rectified, however, sufficient current flows throughthe winding of said relay to cause it to attract its armature. The relayDR then closes a. circuit through the energizing windings 16 and 17 ofthe hang-over relay HR and the suppressor relay SR respectively. Thehang-over relay HR, upon being energized, closes a circuit through thelocking winding 18 of the suppressor relay so'that the suppressor relaywill be, maintained energized as long as the hang-over relay HR isenergized. The hang-over relay HR is provided with an auxiliary Winding19 which. is closed through adjustable resistance 20. By suitablyadjusting the resistance the hang-over relay HR may be made as slow torelease as desired and consequently it may be made to release itsarmature at any definite time after the voice currents have ceased toenergize the detector relay DR. Consequently, the action of the echosuppressoris such that'when voice currents are transmitted over the linewith which it is connected it will almost immediately operate thecontact 21 of the suppressor relay SR to disable the line transmittingin the opposite direction and this condition will continue for apredetermined interval (depending upon the adjustment of the resistance20) after the voice currents have ceased.

In the arrangement of Fig. 1 a small interval of time M must elapsebetween the arrival of the voice waves at the input .terminals of thesuppressor and the closure of the contact of the suppressor relay whichshort-circuits the opposite line. This make interval is due to theself-inductance of the relays and the inertia of the armatures, but maybe made very short. An interval E, elapses between the arrival of thevoice wave at the input terminals of. the suppressor and the arrival ofthe echo at the point where the suppressor relay short-circuits theline. (The subscripts 1 and 2 applied. to the symbols used in thisspecification indicate that the symbols refer to conditions in Figs.land 2respectively.) The echo interval E is equal to the time oftransmission of an electrical impulse from the suppressor to the ,end ofthe fourwire circuit and back. A different interval B for the breakoperation of the suppressor elapses between the instant at which thelast voice wave 'passes the input terminals of the suppressor and theinstant at which the suppressor relay con tacts open. In order that thesuppression of the echoes may be complete it is necessary that the maketime M be shorter than the echo time E Also the break time'B must belonger than the echo time E,. In other words, the relation B, E, M mustexist in order that thecontacts of the suppressor relay "close beforethe arrival of the echo and remain closed until the echo has ceased.

In the case of a four-wire circuit involving extra light loaded lines athousand miles long, for example, the interval E will be about 0.05seconds where the suppressors are located at or near the midpoint of thecircuit. There is no difliculty in making the action of tho suppressorrelay quick enough to disable the opposite circuit in the required timeeven though the required time be very much shorter than that abovegiven, but a special delay feature must be used to preventprematureopening of the short-circuiting contacts. This feature is, asalready stated, provided by the adjustable resistance 20 for controllingthe release ofthe hang-over relay. By this adjustment the time B may bemade as small as possible for any given value of E In order for thelistener to break in to interrupt-or reply to the speaker there must beintervals during which the suppressor relay relaxes and clears the pathfrom listener to speaker. Because of the delay mentioned above theremust be intervals of approxi-' mate silence of finite length in thespeakers speech if the listener is to have an opportunity to break in. g

If it could be assumed that the echo suppressor arrangement was providedwith a mechanism whereby the opening of the short-circuiting contactsand the closing of said contacts were made mechanically en tirelyindependent of each minimum interval I in the speakers speech, whichwould just permit the suppressor relay contacts to open, would be Thiswill be readily apparent fromthe curves of Fig. 4. In this figure it isassumed that 25 represents the rectified voice current operating therelay. 26'indicates an interval of vocal silence followed by furtherrectified voice currents indicated at 27. Since the relay remainsoperated an interval B after the current 25 has ceased it will be notedthat the curve 28, indicating the period during which the relay isoperated, continues for its full value for a period B after the curve25- drops to zero. On the other hand, since an interval M must elapseafter the current has again commenced to flow, as indicated at 27, thecurve 29. indicating that the relay is-again energized,

other, then the I does not begin immediately after the curve 2? begins.In other words, the period during which the relay is unoperated will beshorter than the period of vocal silence 26. Suppose, now, the intervalbetween curves 28 and 29 be eliminated, as indicated at 30 and 31. Thenfrom curves 32 and 33 it will be at once apparent that the shortestperiod during which no current flows through the relay withoutpermitting the contacts to open will be equal to B," M.

The above results are based upon the assumption that the opening andclosing of the contacts are independent. In practice this is not true,however, and a greater interval will be necessary, after the arrival ofthe voice currents, to produce a closure when the contacts are open thanwill be required to insure that they will'not open when they are alreadyclosed. For this reason a greater interval of silence than that given byE uation (1) is required and the following re ations holdsapproximately.

'In the operation of acircu-it of this character there are two timeelements to be considered, first, the time WlllCll" must elapse "beforethe listener can talk in the opposite direction after the voice currentstransmitted from the speaker have ceased and. second,

sive control of the circuit but merely to prevent the echoes from beingtransmitted over the part of the circuit normally used by the otherparty. on the other hand, it is extremely desirable that the firstperiod he made as short as possible in order that the listener canreplyt p r'omptly after the speaker has ceased talking. a

In order that the listener may take control it is. necessary that thecontacts of the suppressor relay remain open for the suppressor workingin the ;opposite direction to operate. If the two parts of thesuppressor SIV and. SE are alike there will he required in addition tothe interval I,. already referred to. an additional interval M for theshortest pause in the talkers speech required to permit the listener tobreak in. If the total length of the necessary pause in the talkersspeech is L the folowing relations hold for Fig. 1.

In the case of moderately long circuits there isno difficulty in makingthe closing interval M of the suppressor relay short com-- pared withthe time of propagation from the middle of the circuit to the end andback. In accordance with the present inlong enough SV of the echo sup-*quick to disable the return circuit before:

the echo arrives at the disabling point.

Where the suppressor apparatus islocated near the end of the circuit theecho time E of the arrangement of Fig. 2 wouldbe small and the break orreleasing time B, could be reduced to a corresponding value.

In this case. since B is short, the interval I. (compare Equation 2)required to per mit the relay contacts to open and allow a disturbanceto pass from the listener back to the talker. is short and the listenernot only has an increased number of opportunities to interrupt thespeaker but is enabled to reply after the speaker has ceased talking. Onthe other hand, since the two parts of the echo suppressor are separatedfrom each other. the listener cannot so easily and quickly gainexclusive control of the circuit, although as already stated, the latterconsideration is-of minor consequence in the,

to the speaker a minimum intervaldesign of echo suppressors. An intervalot time T is required for the transmission of an impulse from one partofthe echoisup pressor to the other, consequently, the total interval ofsilence required to permit the listener-to completely cut off thetalker-is made up ot 13,, the time for the suppressor relay nearer tothe listener to release,,T, the

intervalirequired for the listeners voice to travel to the suppressornearer the talker, M the interval required to operate this suppressornearer the talker and a :further interval T to insure that there will beno impulses traveling -between the suppressors from the talker whichwill reach the suppressor nearer the listener and operate it. I'Iowever.an impulse of speech from the talker lasting less than M seconds wouldbe insuflicient to operate the suppressor nearer to the listener andprevent him from taking control. This would shorten the interval ofsilence required in the speech of-the talker necessary to enable thelistener to take control by M seconds and we have the relation:

dle of the circuit, that is. in Fig. 1 the suppressors are both at themiddle of the circuit and in Fig. 2 they are equally spaced fromtheends, and assuming further that B, and B are made as short as possible,

then 1 B1 E1 and B2 E: but E E T (7) 1 hence B B T (8) from whichsubstituting in Equation (3) r ,L,=B,+M=B +M+T (9) from (4) and (9) L=L,+TM (10) From Equation (8) it is evident that the 2 arrangement shownin Fig. 2 requires an interval in the speakers speech shorter by T.

" It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated without departin from the spirit of the invention as definedin the following claims.

\Vhat is claimed is:

1. In a transmission system, two'distantly separated stations,transmission paths interconnecting said stations, one of said pathstransmitting in one direction and the other I of said pathstransmittingin the opposite direction, said paths extending at least in part overdifferent systems of conductors and each of said paths being normallyoperable for transmission in their respective 0 directions when nosignaling currents are being transmitted, means at said terminalstations whereby currents transmitted over one path. Wlll cause echocurrents to be transmitvted 1n the op osite directions over the other455 path, means or preventing the transmission of such echo currentscomprising two units each responding to voice currents transmitted inone pathfor disabling the path transmitting in the opposite direction,one of said units being associated with one path at a point near thestation opposite that from which operating'voiceycurrentsare'transmitted and where the paths extend over difi'erent systems ofconductors, and the otherunit being associated with the other path at apoint near the other station and where the paths extend over difierentsystems of conductors. I

2. In a transmission system, two distantly separated stations,transmission paths interconnecting said stations, one of said pathstransmitting in one direction and the other of said paths transmittingin the opposite direction, said paths extending at least in part overdifferent systems of conductors and each of said paths being normally0perable for transmission in their respective directions when nosignaling currents are being transmitted, means at said terminalstations whereby currents-transmitted over one path Will cause echocurrents to be transmitted in the opposite directions over the otherpath, means for preventing the transmission of such echo currents, saidmeans comprising two units one associated with each path at a pointwhere the paths extend over difl'erent systems of conductors, each unitincluding means responsive to currents transmitted over the path withwhich fit is associated for disabling the path transmitting in theopposite direction over a separate arranged one at a point' near theterminal station opposite that from which 0 crating voice currents aretransmitted, and t at a point near the other terminal station.

3. In a transmission system, two distantly separated stations, afour-wire circuit for interconnecting said stations, said four-wirecircuitcomprising two independent one-way paths extending over differentsystems of conductors, one being used for transmission in one directionbetween the terminals and the other being used for transmission in theopposite direction between the terminals, said paths being normallyoperable for transmission in their respective directions when nosignaling currents are being transmitted, means for interconnecting saidpaths at the terminal stations with'terminal transmission systemswhereby currents transmitted over one path will cause echo currents tobe transmitted in the opposite direction over the other path, means forpreventing the transmission of such echo currents, said means comprisingtwo separate units, one of said units be associated with one path nearthe terminal station opposite that from which operating voice currentsare transmitted, and the other unit being associated with the other pathnear the other terminal station and each unit including means responsiveto voice currents transmitted from the distant station over the pathwith which it is associated for disabling the path transmitting in theopposite direction.

' In testimony whereof, I have signed my name to this specification this2nd day of August,' 1923.

' OTTO B. BLACKWELL.

e other system of conductors, and said units being

